The manufacture of semi-conductor devices such as diodes and transistors typically requires the deposition of dielectric materials such as polycrystalline silicon, silicon nitride and silicon dioxide on the surfaces of thin silicon wafers. The thin layer deposition of these materials involves rapid heating and cooling cycles in an electrically heated furnace (or "diffusion process tube") at temperatures typically ranging from 250 to 1000 C. When dielectric precursor gases are fed into a diffusion process tube heated to these temperatures, the gases react and deposit the dielectric reaction product on the surface of the silicon wafer.
During the deposition step, the silicon wafers are supported is in vertical or horizontal boats placed within the process tube. The wafer boat and process tube are typically made of a material which has excellent thermal shock resistance, high mechanical strength, an ability to retain its shape through a large number of heating and cooling cycles, and which does not out-gas (i.e., introduce any undesirable impurities into the atmosphere of the kiln during firing operations). One material which meets these requirements is silicon carbide. For the above-mentioned application, silicon carbide diffusion components such as boats, paddles and process tubes are typically pre-coated with the dielectric selected for deposition.
When the silicon wafers are processed in a boat, it is naturally desirable that each wafer in the boat be exposed to identical gas concentration and temperature profiles in order to produce consistent product. However, the typical fluid dynamic situation is such that consistent profiles are found only in the middle of the boat while inconsistent profiles are often found at the ends of the boats, resulting in undesirable degrees of dielectric deposition upon the end-wafers which render them unusable.
One conventional method of mitigating this "end-effect" problem is to fill the end slots of the boat with sacrificial ("dummy") wafers made of silicon. However, it has been found that silicon wafers are expensive, extensively out-gas, warp at high process temperatures, flake particles, and have a short useful life span.
Another conventional method of mitigating the "end-effect" problem is to fill the end slots of the boat with dummy wafers made of alternative materials. For example, one investigator placed SiC-coated carbon wafers having the exact dimensions of the neighboring silicon wafers in the end slots. However, these wafers were found to break apart, contaminating the furnace with carbon particles. Another investigator proposed using CVD monolithic silicon carbide as a dummy wafer. However, this material is known to be very expensive. One prior proposal for producing silicon carbide wafers is a freeze casting approach which produces a green silicon carbide billet having a thickness of at least about 25 mm which is recrystallized and then sliced to a commercially useful thickness. However, it has been found that the freeze casting process produces significant porosity in the wafer (on the order of 40 v/o, with 15 percent of the pores larger than 25 .mu.m). These large pores make it difficult to completely precoat the wafer with the dielectric and make the deposition process very expensive. JP Patent Publication No. 5-283306 ("the Toshiba reference") discloses forming a commercially useful wafer by grinding down a 2 mm thick siliconized silicon carbide disc to a thickness of about 0.625 mm, and then CVD coating the disc with an alumina-silica coating. However, the grinding, siliconization and CVD steps are expensive, particularly so in the low temperature (less than 1000 C.) applications where silicon infiltration is not required to prevent the oxidation of the silicon carbide.
Therefore, it is the object of the present invention to provide an inexpensive silicon carbide dummy wafer which possesses the dimensional, physical and mechanical properties required for use in applications with temperatures less than about 1000 C.